Glass optical fibers are replacing conventional insulated electrical conductors in communications cables because the fibers, guiding modulated light signals instead of electrical signals, offer increased channel capacity or greater data transmission rates through long cable lengths without repeaters. Conventional electrical cables have signal transmission bandwidths that are limited to about 100 KHz through lengths up to 30,000 feet. By contrast, optical fibers can transmit signal bandwidths up to 100 MHz through this length. This corresponds to 3 orders of magnitude larger data transmission rate.
Glass optical fibers have two properties which make it difficult to successfully incorporate them into cables which will be stretched when used, or which will be used under water, particularly at high pressure and/or high temperature, such as one finds down in an oil well drill hole. These properties are static fatigue degradation and microbending loss.
Silica glass fibers have small cracks (microcracks) on their surfaces. The depth of these microcracks can increase through a stress-accelerated chemical reaction, between the silica glass and moisture, called static fatigue. The tensile strength of the glass fiber decreases substantially as the microcracks increase in depth. Glass is an elastic material with a high Young's modulus. Strain in a glass optical fiber generates tensile stress and results in static fatigue. Thus, glass optical fibers are not suitable for use under high strain (greater than 0.5%) in the presence of moisture over extended periods of time. No plastic can provide adequate protection since water diffuses through all of them to some degree.
Loss of light through small bends in the fiber (microbending loss) is here described. Optical fibers transmit light signals by the principle of total internal reflection. This principle depends upon the light rays being totally reflected back into the core region each time they impinge upon the core to cladding interface of the optical fiber. Total internal reflection can only occur when the angle of incidence between the rays and the core to cladding interface is below a certain critical value. Bending of an optical fiber causes some of the light which is propagating in the fiber core to impinge upon the core to cladding interface at angles of incidence greater than the critical value and to be refracted out of the optical core and lost. The amount of the light that is lost becomes greater as the effective diameter of the bend becomes smaller. When the bending of the optical fiber is caused by deflection due to local lateral forces, the resulting decrease in signal strength (and decrease in the length of cable which can be used) is called microbending loss. When an optical fiber is deflected by a local inhomogeneity, such as a lump in its coating layers, the effective diameter of the bend depends upon the local strain the fiber is under. Generally, the fiber will bend to a smaller effective diameter as the strain level it is under increases. Consequently, higher strain levels result in higher levels of microbending loss.
Microbending losses in fibers are greatly reduced by first coating the fibers with a soft elastomer such as silicone rubber, and encasing this buffered fiber in a rigid jacket which can withstand external forces. However, plastics such as silicone rubber lose their integrity under conditions encountered in harsh environments such as at the bottom of a deep well, i.e., when subjected to hot brine at pressures up to 20,000 psi and temperatures up to 500.degree. F.
A possible solution to these problems is described in European Pat. No. 0 48 674, entitled Fiber Optic Cable and Core, and issued to the Schlumberger Company. In this design a metal strip is wrapped helically around a core containing optical fibers, and then a second metal strip is wrapped around the first in such a way as to overlap the abutting edges of the first helix. Then the strips are soldered together to hermetically seal the tube so formed. The biggest drawback to this design is that when the tube is bent over a sheave or spool the seal is subjected to the longitudinal stress on the outside of the bend, and will give way and open up. In addition, sealing a spiral seam with a material other than solder or glue, e.g. welding, would be prohibitively expensive.
Thus, it would be highly desirable to have a hermetically sealed tube which is formed so as to avoid the tendency to open up when bent over a spool and/or subjected to longitudinal stress. It would also be highly desirable to have a process, and the product resulting therefrom, of producing a hermetically sealed metal tube which can be formed around a stranded and buffered optical fiber without stretching the fiber or introducing microbends.
Standard methods of forming welded metal tubes start with a flat strip which is formed by bending into a tube shape with appropriate rollers and butt-welded at the edges. This fabrication process requires the strips to be thin and annealed. If the material to be worked is a potentially high strength steel or other alloy, it can be work hardened by passage through a drawing or sinking die. A sunk metal tube is defined as a tube the diameter of which has been reduced by passage through a die without using an internal support. The strip must be configured so that the final metal tube will have a sufficient wall thickness to preclude buckling under pressure. Also, the tube must have a sufficiently high yield strain to avoid inelastic stretching when bent over a sheave or cable reel. The pressure buckling can be eliminated by building up several welded layers of increasing inside and outside diameter as has been successfully performed around conventional insulated wires. However, glass fibers cannot stand the stretch of the drawing or sinking process to work harden the metal. Thus, a method of tube forming which avoids the drawing or sinking process but nevertheless results in high tensile yield strengths is desirable.